Both the Achilles RS-156 2010 and the Achilles SU-18 2009 are inflatable designs, both built in inflatable, so the buying decision usually comes down to a handful of practical questions: how many people are you putting on the water, how far do you trailer, and what does your tow vehicle weigh?
On paper these two are close siblings in the size department - Achilles RS-156 2010 at 15,5 ft versus Achilles SU-18 2009 at 18,0 ft. Weight tells a clearer story for trailering families: the Achilles SU-18 2009 tips the scales at 440 lbs - 288 lbs less than the Achilles RS-156 2010 at 152 lbs. That difference is meaningful if you're working within a half-ton or three-quarter-ton truck's tow rating, especially once you factor in a motor, gear, and fuel.
The Achilles SU-18 2009 tops out at 90 hp. Engine specs for the Achilles RS-156 2010 aren't listed - confirm with a dealer before selecting an outboard.
For family outings this is probably the sharpest distinction between the two. The Achilles RS-156 2010 is rated for 12 passengers, while the Achilles SU-18 2009 caps at 11. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Achilles RS-156 2010 could be the deciding factor.
Bottom line: Choose the Achilles RS-156 2010 if your priority is putting more people on the water - it handles 12 passengers and at 15,5 ft it has the deck room to back that rating up comfortably. The Achilles SU-18 2009 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 11 that costs less to run day-to-day.